docs: new design for the documentation site (#5934)

* docs: new design for the documentation site

Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono
served by the site (upright and real italic), brand colours, blue header band
with the syntax pattern, wordmark in the header, new home page (hero, cards,
animated feature tour), 404 page, polish for tabs, search, footer, focus
rings, tables and code blocks, and theme features (code copy, previous/next
footer, TOC follow, search suggestions).

Draw the number railroad diagram, the parser callback events and the iterator
illustrations as inline SVGs that follow the light and dark scheme. The
generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow
(json-light.svg, json-dark.svg, replacing json.gif on the home page) is built
by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become
hexagons, which need far less room.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: add figures, table captions and a macro chooser

- Figures with numbered captions: SAX stepper, json_view layout and edit
  stepper, exception map, JSON pointer and patch walkers, QA pipeline,
  scaling and binary size charts, roadmap lanes, release timeline,
  integration decision tree and namespace builder
- Table captions (nj-table-caption) on 9 pages
- Compile times: modules and PCH charts, filesystem section, re-measured
  Figure 1, links between features/modules.md and compile_times.md
- Interactive macro chooser for features/arbitrary_types.md
- Link check: exclude URLs that answer the plugin with 400/401/419 or are
  dead (customers.md)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: macro chooser generates adl_serializer for non-default-constructible types

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: cross-link concept pages and API pages in See also sections

Add See also links in both directions between the concept pages (features,
home, integration) and the API pages, write API links in code font, give every
entry a comment after a dash, and show the callback event figure on the parser
callbacks page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: address review comments on the new design

- Home: add a "Latest release" button
- Sponsors: update the list and group it by tier
- Features: list all feature pages on the index page
- Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline
- Exceptions: prefix all example messages with the exception name
- API: no code font for format names in the See also links of the to_/from_ pages
- Merge Patch: show the document before, the patch, and the result
- Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -15,8 +15,8 @@ basic_json_view operator[](IntegerType idx) const;
 basic_json_view operator[](const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
-   the [Notes](#notes) below) -- or a [discarded](is_discarded.md) view if there is no such member.
+1. Returns the value of the object member with key `key` — the first one, should the key occur more than once (see
+   the [Notes](#notes) below) — or a [discarded](is_discarded.md) view if there is no such member.
 2. Returns the array element at index `idx`, or a [discarded](is_discarded.md) view if `idx` is out of range. The
    template accepts every integer type except `#!cpp bool` and `#!cpp std::size_t` (`#!cpp int`, `#!cpp unsigned`,
    `#!cpp long`, `#!cpp std::int64_t`, ...) and forwards to the `size_type` overload, so that an integer argument is
@@ -55,11 +55,11 @@ Strong guarantee: there are no changes to the view or the document it refers to.
 ## Exceptions
 
 1. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an object and
-   not [discarded](is_discarded.md) --
+   not [discarded](is_discarded.md) —
    the same exception, with the same message, that the **const** overload of
    [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a string argument on a non-object value.
 2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an array and
-   not [discarded](is_discarded.md) --
+   not [discarded](is_discarded.md) —
    the same exception, with the same message, that the **const** overload of
    [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a numeric argument on a non-array value.
 3. Throws the same exceptions, with the same messages, that the **const** overload of
@@ -81,7 +81,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
    another, in document order, stopping at the first match. Each comparison first checks the
-   key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a
+   key's length — already known from the index, without reading the key bytes — before comparing its content, so a
    key of a different length than `key` is rejected without touching the source text.
    Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
    constant time on average.
@@ -97,7 +97,7 @@ Unlike `BasicJsonType::operator[]`, which is undefined behavior (guarded by a
 [runtime assertion](../../features/assertions.md)) for a missing key on a **const** value, this operator returns a
 safe, testable result for a missing key or an index out of range: a [discarded](is_discarded.md) view, which is
 tested with [`is_discarded`](is_discarded.md).
-There is also no non-const overload that inserts a missing key or extends an array -- a view never modifies the
+There is also no non-const overload that inserts a missing key or extends an array — a view never modifies the
 document.
 
 !!! info "Chained access"
@@ -126,8 +126,8 @@ document.
 
     Overload 3 walks `ptr` one reference token at a time, starting at this value, the same way [`at`](at.md) and
     [`contains`](contains.md) do. It only ever returns a discarded view where the **const** overload of
-    `BasicJsonType::operator[]` would be undefined behavior for the same pointer -- a missing object member or an
-    out-of-range array index -- and still throws for every other way `ptr` can fail to resolve. See
+    `BasicJsonType::operator[]` would be undefined behavior for the same pointer — a missing object member or an
+    out-of-range array index — and still throws for every other way `ptr` can fail to resolve. See
     [`at`](at.md#exceptions) for the checked version, which throws in every case instead, and
     [`contains`](contains.md) for a version that never throws.
 
@@ -137,7 +137,7 @@ document.
 
     The example below reads a couple of fields out of a batch of user records without ever materializing a full
     `BasicJsonType` value for the batch. `operator[]` is used both to look up an optional object member and to index
-    into an array -- in both cases, a missing value comes back as a discarded view that can be tested with a plain
+    into an array — in both cases, a missing value comes back as a discarded view that can be tested with a plain
     `#!cpp if`, instead of relying on undefined behavior.
 
     ```cpp
@@ -168,9 +168,9 @@ document.
 
 ## See also
 
-- [at](at.md) - access specified element with bounds checking (throws instead of returning a discarded view)
-- [front](front.md), [back](back.md) - access the first or last element
-- [find](find.md), [contains](contains.md) - look up a member without throwing
+- [`at`](at.md) - access specified element with bounds checking (throws instead of returning a discarded view)
+- [`front`](front.md), [`back`](back.md) - access the first or last element
+- [`find`](find.md), [`contains`](contains.md) - look up a member without throwing
 - [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) - the corresponding function of `basic_json`
 - [`json_pointer`](../json_pointer/index.md) - JSON pointer type used by overload 3